Propylene polymers have been conventionally molded by various molding methods and the molded articles are applied to extensive uses.
The propylene polymers are generally prepared using a catalyst comprising a transition metal compound and an organoaluminum compound, i.e., so-called Ziegler catalyst.
Propylene polymers prepared by the use of a titanium catalyst containing a halogen-containing titanium catalyst component among the Ziegler catalysts are excellent in moldability and rigidity, but they have such problems that they are poor in tensile elongation at break. Moreover, the titanium catalyst causes a large amount of a catalyst residue in the resulting polymer because of low polymerization activities, and hence the molded article is sometimes colored or deteriorated in sanitariness.
On the other hand, propylene polymers prepared by the use of a metallocene catalyst containing a transition metal compound catalyst component such as zirconocene are excellent in tensile elongation at break, but they have such problems that they are poor in moldability and rigidity (flexural modulus). As for the metallocene catalyst, however, the amount of the catalyst residue is small because of high polymerization activities, and the molded article is never colored and is good in sanitariness.
Though the characteristics required for the propylene polymers vary depending on the molding methods or uses, generally required are moldability, heat resistance, mechanical strength, high tensile elongation at break, impact resistance, etc. For satisfying these requirements, researches on various compositions such as a composition obtained by blending two or more kinds of propylene polymers and a composition obtained by blending a propylene polymer and other synthetic resin have been made.
For example, blending of two kinds of propylene polymers which are different in the molecular weight has been carried out in order to improve physical properties of the propylene polymers prepared by the use of a titanium catalyst. However, when two kinds of propylene polymers produced by the use of a titanium catalyst are blended to prepare a propylene polymer composition, the tensile elongation at break of the resulting composition is markedly lowered, though the moldability thereof is improved.
Further, adding of a soft polymer to a propylene polymer which is prepared by the use of a titanium catalyst has been carried out in order to improve the tensile elongation at break and the impact resistance of the propylene polymer. The soft polymer used therefor is, for example, an ethylene/propylene random copolymer prepared by the use of a titanium catalyst or a vanadium catalyst. However, even if the propylene polymer prepared by the use of a titanium catalyst is blended with the ethylene/propylene random copolymer prepared by the use of a titanium catalyst or the like, the resulting composition is not sufficiently improved in the tensile elongation at break and the impact resistance.
As described above, the conventional propylene polymer compositions are not always satisfactory in the properties such as heat resistance, mechanical strength and tensile elongation at break.